Originally developed for IBM PC Compatible computers, the Basic Input/Output System (BIOS), also known as the System BIOS, is a de facto standard defining a firmware interface. The BIOS is boot firmware, designed to be the first code run by a PC when powered on. The initial function of the BIOS is to identify, test, and initialize system devices such as the video display card, hard disk, and floppy disk and other hardware. This is to prepare the machine into a known state, so that software such as an operating system stored on compatible media can be loaded, executed, and given control of the PC. This process is known as booting, or booting up, which is short for bootstrapping.
BIOS programs provide a small library of basic input/output functions that can be called to operate and control the peripherals such as the keyboard, text display functions and so forth. When a system is powered on, BIOS checks system configuration settings and loads programs that act as translators between the hardware components and the operating system into memory. For example, when a user presses a key on the keyboard, the signal is sent to a keyboard interrupt handler, which tells the processor what it is and passes it on to the operating system.
BIOS was developed for the IBM PC at a time when processors functioned in a 16-bit processor mode and addressable memory was limited to one megabyte, and the code reflected IBM PC AT hardware dependencies. Operating systems later developed for 32-bit processors began to include device drivers to handle I/O rather than rely on invoking the 16-bit runtime interface provided by BIOS. These device drivers are often provided by platform firmware and are loaded into memory during BIOS initialization prior to loading the operating system. The Extensible Firmware Interface (EFI) is a specification that defines a software interface between an operating system and platform firmware. EFI defines boot services, which include text and graphical console support on various devices, bus, block, and file services, and runtime services, such as date, time, and NVRAM services. More information about the EFI specification is provided at the URL developer-intel-com/technology/efi/main_specification.htm (where URLs are described herein by omitting the ‘http://’ prefix and replacing the ‘.’ character with the ‘-’ character to avoid an active hyperlink from within this document).